Liquid-liquid extraction has been used earlier in the metallurgical industry typically in the processing of solutions with a weak valuable metal content. Many large copper and uranium recovery extraction plants fall into this category. With regard to copper, however, the situation is changing, because the extraction feed solutions are becoming noticeably stronger with the pressurised concentrate leaching processes coming into operation. Likewise some cobalt and zinc extraction processes also treat strong feed solutions. Nevertheless, the size of the equipment, particularly in the case of copper, will generally remain large, also in the new pressurised leaching processes.
In all extraction processes a valuable metal-containing aqueous solution is brought into contact with an organic solution in the extraction mixing section, forming a dispersion of two solutions that are insoluble in each other. The solutions in the dispersion are separated from each other in the separation section of extraction, where the solutions separate from each other into two layers with a dispersion band remaining between them. During the mixing stage, either one or more of the valuable metals in the aqueous solution is transferred to the organic phase, from which the valuable metals are recovered by stripping the aqueous solution. Extraction is performed in an equipment, where the mixing and separation sections are either located one on top of the other (column) or in series on more or less the same level horizontally. Almost always in cases when large-scale extractions of weak solutions are concerned, such as copper extraction, the equipment is positioned in an essentially horizontal position. When we refer to extraction hereinafter, the term is used for various arrangements, but essentially equipment in the same level.
The separation section in extraction is conventionally rectangular in cross-section or in some special cases square. For instance in U.S. Pat. No. 6,132,615 the settler described is of rectangular shape. The settler is equipped with several picket fences.